Why I Always Check the Wet Bulb Temperature Before a Summer Run

Working out in the heat can be merely unpleasant, or it can be dangerous, depending on the day—and there’s a good way to tell what kind of day it is. This summer, I’ve checked the wet bulb temperature before nearly every run I’ve been on. This number is more important to me than the actual temperature, or even the “feels like” temperature, because it helps me gauge the safety of exercising outdoors. I’ll delay or call off a workout if the wet bulb temp is too high.

I’ve always known that heat makes summer running feel harder, but this year I’ve been experimenting with a Core body temperature sensor to study how my body responds to the heat. (It’s a cool gadget—here’s my review so far.) I was seeking out hot temperatures in the spring, the better to get my body adapted to hot weather running. But now it’s mid-July, and even my hard-won heat adaptation isn’t enough to keep me safe on the hottest, most humid days of a heat wave.

To help you stay safe while exercising during the hottest part of the year, allow me to explain what wet bulb temperature means, and what numbers to watch out for when gauging whether you should attempt that outdoor workout.

It’s the heat and the humidity

When it’s hot, you can often still exercise safely, and training in the heat forces your body to adapt to it. Even so, our bodies have their limits. There really are temperatures and conditions that make it dangerous to exercise for very long. Most importantly, it’s not just about the temperature. Humidity matters, too. A hot and dry day is a lot easier for your body to handle than a hot and humid day.

I’ll explain below how to use the wet bulb globe temperature to tell when the combination of heat and humidity makes conditions dangerous to work out. But to give a few examples, if you’re trying to avoid wet bulb temperatures above 82 (a common recommendation), these scenarios would all be “too hot” for a long or intense run:

• 80 degrees would be too hot at 70% humidity

• 90 degrees would be too hot at 30% humidity

• 100 degrees would be too hot at 10% humidity

If you’re used to the heat, you can stand to work out at higher temperatures, as we’ll see. But let’s look first at what “wet bulb” temperature even means.

How to use the wet bulb globe temperature

Instead of just checking the temperature forecast for the day, the best way to figure out how the heat will affect your body is to look at a metric called wet bulb globe temperature, or WBGT. The wet bulb globe temperature comes from the idea that if you wrap the bulb of an old-fashioned thermometer in wet fabric, the thermometer will then give a reading that tells you how much the air can cool you down. (I’m simplifying; the actual process is more complicated and involves three thermometers.)

This is important because one of our body’s main cooling mechanisms—sweating—can’t work well when the humidity is too high. Sweat cools us down by evaporating off our skin, but humidity interferes with evaporation. The WBGT measures how well evaporation can cool off a wet surface in addition to considering the temperature itself.

Fortunately, you don’t need a thermometer and a wet cloth to calculate wet bulb temperature: You can approximate it by looking up the temperature and humidity on a chart, or do what I do, and use the Carrot Weather app to see it in a forecast. WeatherFX (iPhone, Android) is another app that reports wet bulb temps directly.

With Carrot, I can see the WBGT hourly throughout the day. Interestingly, it’s often more humid during the cooler parts of the day, so the wet bulb temp doesn’t always change much from morning to afternoon. I’ll still opt for evening runs when I can, since the sun is often over the horizon and not beating down on me directly. But I can use the day’s wet bulb temperature to decide if I need to shorten a run or maybe skip it entirely.

How to adjust your training based on the wet bulb temperature

Now that you have the WBGT, you need to decide what to do about it. U.S. Soccer has a map that divides the country into three regions, with different cutoffs for canceling practice in different areas. (If you live in Texas, they assume you can deal with a bit more heat than if you live in Minnesota.) Similarly, here are guidelines meant for organizers of road races (like marathons). If the WBGT is over 82 degrees, the race should be canceled. Above 73, “extreme caution and slower pace” are strongly recommended.

To give an idea of what that might look like, a WBGT of 82 degrees can happen in 75-degree weather with 90% humidity, 84-degree weather with 50% humidity, or 100-degree weather with 10% humidity.

If you aren’t used to the heat

For a general set of recommendations, here’s what the American College of Sports Medicine recommends for people who are not acclimated to working out in the heat: